The present disclosure relates in general to the laying of elongated members on the bed of a body of water using a laying vessel. The elongated members, in this case underwater pipelines or cables, are composed of strings, each of which is wound on a reel, and is unwound gradually off the reel, from an unreeling station on board a laying vessel, and laid on the bed of the body of water as the laying vessel moves forward. One of the most critical aspects of this laying technique is feeding the strings to the laying vessel once the reel is unwound.
When the reel is mounted in a fixed position on the laying vessel and the string is reeled off completely, the laying vessel returns to the quay to wind the reel with another string of the elongated member. The string is prefabricated at the quay and simultaneously wound onto the reel on the laying vessel. This operating method involves relatively considerable and costly downtime of the laying vessel, which remains substantially idle the whole time the string is being prefabricated.
To reduce downtime of the laying vessel, U.S. Pat. No. 7,544,013 proposes prefabricating the strings on a floating auxiliary structure; winding the string onto a reel on the floating auxiliary structure; and transferring the string to a reel on the laying vessel, unreeling the string off the reel on the floating auxiliary structure, and simultaneously reeling the string onto the reel on the laying vessel. This method reduces the downtime involved in prefabricating the string, but not the time taken to transfer to string from one reel to another.
An alternative transfer method employs releasable reels that can be detached from the laying vessel, winds the reels in an auxiliary structure, and transfers the reels from the auxiliary structure to the laying vessel.
In line with this operating method, PCT Patent Application No. WO 2011/105894 proposes prefabricating a number of reels; loading the reels onto the laying vessel; and positioning the reels successively at the unreeling station. The wound and unwound reels are loaded and unloaded respectively using cranes, which makes it difficult to position the reels accurately. It should be appreciated that these reels weigh thousands of tons, so handling them is a complex and also hazardous job, especially when they are suspended from a crane. A load suspended from a cable tends to swing, thus making it relatively difficult to position, and also endangering any nearby personnel or structures. Moreover, handling the reels is made even more difficult by the crane being mounted on a floating unit. That is, floating units equipped with cranes are invariably massive, precisely to ensure greater stability of the crane.
EP Patent No. 302,038, PCT Patent Application No. WO2009/077711, U.S. Pat. No. 7,927,040, U.S. Pat. No. 4,117,692, PCT Patent Application No. WO 2011/105894, EP Patent No. 2,591,992, and PCT Patent Application No. WO 2013/006041 describe other technical solutions adopted in this field, but none are fully satisfactory in reducing both downtime of the laying vessel, and endangerment of on-board personnel.
Even documents concerning the transfer of cargos or containers such as U.S. Published Patent Application No. 2011/170,988 French Patent No. 2,268,686, and French Patent No. 2,120,495 from two vessels or from a dock to a vessel do not provide adequate and prompt solutions to the drawbacks of this prior art.